The present invention relates to CMP cleaners for cleaning semiconductor wafers after chemical mechanical polishing (CMP). More particularly, the present invention relates to purge bars for purging excess cleaning fluid from a wafer after a CMP cleaning to reduce the chemical concentration in the environment of the cleaner.
In the fabrication of semiconductor devices from a silicon wafer, a variety of semiconductor processing equipment and tools are utilized. One of these processing tools is used for polishing thin, flat semiconductor wafers to obtain a planarized surface. A planarized surface is highly desirable on a shadow trench isolation (STI) layer, inter-layer dielectric (ILD) or on an inter-metal dielectric (IMD) layer, which are frequently used in memory devices. The planarization process is important since it enables the subsequent use of a high refluid lithographic process to fabricate the next-level circuit. The accuracy of a high refluid lithographic process can be achieved only when the process is carried out on a substantially flat surface. The planarization process is therefore an important processing step in the fabrication of semiconductor devices.
A global planarization process can be carried out by a technique known as chemical mechanical polishing, or CMP. The process has been widely used on ILD or IMD layers in fabricating modern semiconductor devices. A CMP process is performed by using a rotating platen in combination with a pneumatically-actuated polishing head. The process is used primarily for polishing the front surface or the device surface of a semiconductor wafer for achieving planarization and for preparation of the next level processing. A wafer is frequently planarized one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer can be polished in a CMP apparatus by being placed on a carrier and pressed face down on a polishing pad covered with a slurry of colloidal silica or aluminum.
A polishing pad used on a rotating platen is typically constructed in two layers overlying a platen, with a resilient layer as an outer layer of the pad. The layers are typically made of a polymeric material such as polyurethane and may include a filler for controlling the dimensional stability of the layers. A polishing pad is typically made several times the diameter of a wafer in a conventional rotary CMP, while the wafer is kept off-center on the pad in order to prevent polishing of a non-planar surface onto the wafer. The wafer itself is also rotated during the polishing process to prevent polishing of a tapered profile onto the wafer surface. The axis of rotation of the wafer and the axis of rotation of the pad are deliberately not collinear; however, the two axes must be parallel. It is known that uniformity in wafer polishing by a CMP process is a function of pressure, velocity and concentration of the slurry used.
A CMP process is frequently used in the planarization of an ILD or IMD layer on a semiconductor device. Such layers are typically formed of a dielectric material. A most popular dielectric material for such usage is silicon oxide. In a process for polishing a dielectric layer, the goal is to remove typography and yet maintain good uniformity across the entire wafer. The amount of the dielectric material removed is normally between about 5000 A and about 10,000 A. The uniformity requirement for ILD or IMD polishing is very stringent since non-uniform dielectric films lead to poor lithography and resulting window-etching or plug-formation difficulties. The CMP process has also been applied to polishing metals, for instance, in tungsten plug formation and in embedded structures. A metal polishing process involves a polishing chemistry that is significantly different than that required for oxide polishing.
Important components used in CMP processes include an automated rotating polishing platen and a wafer holder, which both in exert a pressure on the wafer and rotate the wafer independently of the platen. The polishing or removal of surface layers is accomplished by a polishing slurry consisting mainly of colloidal silica suspended in deionized water or KOH fluid. The slurry is frequently fed by an automatic slurry feeding system in order to ensure uniform wetting of the polishing pad and proper delivery and recovery of the slurry. For a high-volume wafer fabrication process, automated wafer loading/unloading and a cassette handler are also included in a CMP apparatus.
As the name implies, a CMP process executes a microscopic action of polishing by both chemical and mechanical means. While the exact mechanism for material removal of an oxide layer is not known, it is hypothesized that the surface layer of silicon oxide is removed by a series of chemical reactions which involve the formation of hydrogen bonds with the oxide surface of both the wafer and the slurry particles in a hydrogenation reaction; the formation of hydrogen bonds between the wafer and the slurry; the formation of molecular bonds between the wafer and the slurry; and finally, the breaking of the oxide bond with the wafer or the slurry surface when the slurry particle moves away from the wafer surface. It is generally recognized that the CMP polishing process is not a mechanical abrasion process of slurry against a wafer surface.
While the CMP process provides a number of advantages over the traditional mechanical abrasion type polishing process, a serious drawback for the CMP process is the difficulty in controlling polishing rates at different locations on a wafer surface. Since the polishing rate applied to a wafer surface is generally proportional to the relative rotational velocity of the polishing pad, the polishing rate at a specific point on the wafer surface depends on the distance from the axis of rotation. In other words, the polishing rate obtained at the edge portion of the wafer that is closest to the rotational axis of the polishing pad is less than the polishing rate obtained at the opposite edge of the wafer. Even though this is compensated for by rotating the wafer surface during the polishing process such that a uniform average polishing rate can be obtained, the wafer surface, in general, is exposed to a variable polishing rate during the CMP process.
Recently, a chemical mechanical polishing method has been developed in which the polishing pad is not moved in a rotational manner but instead, in a linear manner. It is therefore named as a linear chemical mechanical polishing process, in which a polishing pad is moved in a linear manner in relation to a rotating wafer surface. The linear polishing method affords a more uniform polishing rate across a wafer surface throughout a planarization process for the removal of a film layer from the surface of a wafer. One added advantage of the linear CMP system is the simpler construction of the apparatus, and this not only reduces the cost of the apparatus but also reduces the floor space required in a clean room environment.
An important challenge in CMP is to produce a clean substrate surface following polishing. Therefore, a primary concern with the use of CMP is the efficient and complete removal of the polishing slurry and other polishing residues and particulates following polishing in order to prevent introduction of defects into the polished product. Ideally, post-CMP cleaning should remove all polishing slurry, polishing residues and particulates in a quick and repeatable fashion without introducing additional defects or damage to the substrate surface. Cleaning procedures following CMP typically include use of a DI (deionized) water rinse and a scrub with a soft rotating brush to remove slurry residue from the surface of the semiconductor substrate. However, use of a DI water rinse alone causes the brush to become loaded with particles, which tend to contaminate other wafers. Accordingly, ammonium hydroxide, hydrogen fluoride, hydrogen peroxide and other chemicals may be used in conjunction with water to clean the wafers.
A typical conventional CMP cleaner for cleaning wafers after chemical mechanical polishing is generally indicated by reference numeral 10 in FIG. 1. The CMP cleaner 10 includes multiple cleaning chambers 12 (three of which are shown), each of which contains a pair of opposing spray nozzles 16, a pair of scrub brushes 20 and a bottom roller 22. The spray nozzles 16 of each cleaning chamber 12 spray a particular chemical cleaning fluid 18, typically including DI water, ammonium hydroxide, hydrogen peroxide or hydrogen fluoride, for example, on opposite surfaces of a wafer 24 as the roller 22 rotates the wafer 24 in the cleaning chamber 12. A lid 14 is provided on each cleaning chamber 12. A wafer transfer mechanism 26, such as a xe2x80x9cwalking beamxe2x80x9d, sequentially transfers each wafer 24 among the multiple cleaning chambers 12 to facilitate cleaning each wafer 24 with a different cleaning fluid 18 throughout the wafer cleaning operation.
One of the problems encountered as the walking beam 26 transfers each wafer 24 among the multiple cleaning chambers 12 is that residual cleaning fluid 18 remaining on the wafer 24 tends to drop on the lid 14 after the walking beam 26 lifts the wafer 24 from the cleaning chamber 12. This undesirably increases the concentration of the cleaning fluid 18 in the environment of the CMP cleaner 10. Accordingly, a device is needed for removing residual drops of cleaning fluid 18 from both surfaces of each wafer 24 before the wafer 24 is removed from each cleaning chamber 12 of the CMP cleaner 10.
An object of the present invention is to provide an apparatus for reducing or preventing dripping or spillage of residual cleaning fluid from an object during or after removal of the object from a cleaning chamber.
Another object of the present invention is to provide an apparatus for removing residual cleaning fluid from an object before or as the object is removed from a cleaning chamber.
Yet another object of the present invention is to provide a method for removing residual cleaning fluid from an object before or as the object is removed from a cleaning chamber.
Still another object of the present invention is to provide an apparatus for removing residual cleaning fluid from a semiconductor wafer before the wafer is transferred from one cleaning chamber to another cleaning chamber in a chemical mechanical polishing (CMP) cleaner.
Another object of the present invention is to provide an apparatus which reduces undesired chemical concentration in an environment surrounding a CMP cleaner.
In accordance with these and other objects and advantages, the present invention comprises purge bars for blowing excess residual cleaning fluid from an object, particularly a semiconductor wafer, before or as the wafer is removed from a cleaning chamber of a CMP cleaner, for example. Typically, a purge bar is mounted on each side of the cleaning chamber for blowing nitrogen or clean, dry air (CDA) against a corresponding surface of the wafer to remove the excess cleaning fluid from the wafer. The purge bars may be connected to a controller for a wafer transfer device which removes the wafer from the cleaning chamber, such that the purge bars are actuated as the wafer transfer device begins to remove the wafer from the chamber.